Citrate, in Collaboration with a Guanidinium Ion, as a Generator of Cubane-like Complexes with a Range of Metal Cations:  Synthesis, Structures, and Magnetic Properties of [C(NH2)3]8[(MII)4(cit)4]·8H2O (M = Mg, Mn, Fe, Co, Ni, and Zn; cit = Citrate)

• Published in: Inorganic chemistry 2006-05, Vol.45 (9), p.3549-3556
• Main Authors: Hudson, Timothy A, Berry, Kevin J, Moubaraki, Boujemaa, Murray, Keith S, Robson, Richard
• Format: Article
• Language: English
• Subjects: Cations; Citrates - chemistry; Crystallization; Crystallography, X-Ray; Guanidine - chemistry; Magnetics; Metals, Heavy - chemistry; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic051779m

Aqueous reaction mixtures containing citric acid, guanidinium carbonate, and a range of metal cations (Mg2+, Mn2+, Fe2+, Co2+, Ni2+, and Zn2+) at room temperature give crystalline products of composition [C(NH2)3]8[(MII)4(cit)4]·8H2O (cit = citrate). In all cases, the crystals are suitable for single-crystal X-ray diffraction studies, which reveal that the compounds are isostructural (space group P42/n; a ∼ 16.2 Å, and c ∼ 11.5 Å). As was intended, cubane-like [M4(cit)4]8- complex anions are present. The individual citrate units are chiral, but each cubane unit contains two of one hand and two of the other, related around an S 4 axis. The cubane units are involved in no less than 40 H-bonding interactions with guanidinium cations and lattice water molecules. Detailed susceptibility and magnetization studies show that the intracluster magnetic coupling within the MnII, FeII, CoII, and NiII cubanes is very weak in all cases with J values of −0.82, −0.43, and −0.09 cm-1 for the Mn, Fe, and Co species, respectively. A two-J model gave the best agreement with the susceptibility and high-field magnetization data for the NiII case, over the whole temperature range studied, and the sign of the parameters, J 12 = −0.3 cm-1 and J 13 = +2.97 cm-1, correlated with the two Ni−(μ3-O)−Ni angles observed in the cluster structure. All members of the 3d-block [M4(cit)4]8- family have spin ground states, S T, of zero, with the higher S T levels just a few reciprocal centimeters away in energy.